Potential technosignature from anomalously low deuterium/hydrogen (D/H) in planetary water depleted by nuclear fusion technology (2411.18595v2)

Abstract: Deuterium-deuterium (DD) fusion is viewed as an ideal energy source for humanity in the far future, given a vast seawater supply of D. Here, we consider long-lived, extraterrestrial, technological societies that develop DD fusion. If such a society persists over geologic timescales, oceanic deuterium would diminish. For an ocean mass and initial D/H that are Earth-like, fusion power use of only $\sim$10 times that projected for humankind next century would deplete the deuterium-hydrogen ratio (D/H) in $\sim$(a few)$\times 10^8$ years to values below that of the local Interstellar Medium (ISM). Ocean masses of a few percent Earth's would reach anomalously low D/H in $\sim10^6$ to $10^7$ years. The timescale shortens with greater energy consumption, smaller oceans, or lower initial D/H. Here, we suggest that anomalous D/H in planetary water below local ISM values of $\sim16\times 10^{-6}$ (set by Big Bang nucleosynthesis plus deuterium loss onto dust or small admixtures of deuterium-poor stellar material) may be a technosignature. Unlike SETI from radio signals, anomalous D/H would persist for eons, even if civilizations perish or relocate. We discuss wavelengths of strong absorption features for detecting D/H anomalies in atmospheric water vapor. These are vibrational O-D stretching at 3.7 $\mu$m in transmission spectroscopy of Earth-like worlds, $\sim1.5$ $\mu$m (in the wings of the 1.4 $\mu$m water band) in the shorter near-infrared for direct imaging by Habitable Worlds Observatory, and 3.7 $\mu$m or $\sim7.5$ $\mu$m (in the wings of the broad 6.3 $\mu$m bending vibration of water) for concepts like the Large Interferometer for Exoplanets (LIFE).

• The paper proposes that anomalously low deuterium/hydrogen (D/H) ratios in planetary water could be a persistent technosignature indicating advanced civilizations using deuterium-deuterium fusion over geologic timescales.
• An advanced civilization using DD fusion could deplete planetary D/H below local ISM levels in millions of years, potentially detectable via high-dispersion spectroscopy of atmospheric water vapor using future telescopes.
• This novel approach offers a robust, persistent technosignature unlike transient pollutants, expanding the search for extraterrestrial intelligence by looking for enduring evidence of advanced energy use.

An Analysis of Deuterium-Deuterium Fusion as a Technosignature in Extraterrestrial Civilizations

The paper "Potential technosignature from anomalously low deuterium/hydrogen (D/H) in planetary water depleted by nuclear fusion technology" by Catling, Krissansen-Totton, and Robinson explores a unique concept for detecting advanced extraterrestrial civilizations through the depletion of deuterium in water via deuterium-deuterium (DD) nuclear fusion. While current searches for technosignatures typically focus on radio signals or atmospheric pollutants, this work introduces the possibility of identifying long-lived extraterrestrial societies by observing an anomalous reduction in the D/H ratio in planetary water, which would imply prolonged usage of DD fusion as an energy source.

Key Findings and Methodological Approach

The authors present a holistic approach to understanding how a civilization employing DD nuclear fusion over geologic timescales would deplete oceanic deuterium, potentially lowering the D/H ratio below that of the local interstellar medium (ISM). Given Earth's oceanic D/H ratio and the estimated energy requirements of such a civilization, they calculate that an advanced society consuming energy at rates approximately ten times that projected for humanity in the next century could deplete the D/H ratio to these anomalous levels within several million years.

Detection of such a technosignature could be facilitated by future telescopes capable of high-dispersion spectroscopy, focusing on critical absorption features in atmospheric water vapor around wavelengths such as 3.7 µm, 1.5 µm, and potentially 7.5 µm. These observations will require careful signal processing and are contingent on the capabilities of upcoming space missions like the James Webb Space Telescope (JWST), the Habitable Worlds Observatory (HWO), and the Large Interferometer for Exoplanets (LIFE).

Implications and Future Directions

This work positions anomalously low D/H ratios as a robust, persistent technosignature, distinct from transient anomalies like trace pollutants. It provides a novel framework for considering how advanced civilizations may manage energy consumption on a planetary scale and introduces the possibility of detecting remnants of such civilizations long after they may have ceased to exist or relocated.

The paper highlights the importance of D/H measurements in understanding not only potential technosignatures but also the chemical evolution of planetary water. Future theoretical developments and observational strategies may extend this research, allowing for the nuanced characterization of exoplanets and their histories.

Conclusion

The authors contribute significantly to the body of research focused on technosignatures by proposing a sophisticated method to detect long-lived, energy-intensive civilizations. The persistent nature of a D/H technosignature offers a valuable asset to astronomers and SETI researchers, proposing an enduring signature of technological activity that increases the scope of detectable extraterrestrial civilizations. This approach adds a substantive dimension to planetary exploration and the search for extraterrestrial intelligence, underscoring the potential longevity and detectability of advanced technological advancements in the cosmos.